The long-term objectives of this study are to continue to establish the developmental course of a group of individuals with sex chromosome aneuploidy identified at birth in an unbiased fashion. Sixty-eight propositi were identified by checking the sex chromatin status of 40,371 consecutive births in two Denver hospitals from 1964 to 1974. Forty-six of these have been followed with their families prospectively since birth in this Family Development Study. Wherever possible, siblings are used as controls. The original propositi will be either in late adolescence or early adult-hood. Gender identity, sexual and reproductive activity, and adaptation to adult life will be documented. Neuromuscular, neuropsychological, psychiatric, and endocrine evaluation will be particularly emphasized. The nature of the interaction between propositi, their families, and their peers will be documented.